1. Field of the Invention
The present invention relates to an electronic component package having a case for housing an electronic component and a lid sealing an opening of the case, to a lid for the package, and a lid material for the lid.
2. Description of the Related Art
Packages for housing various electronic components such as semiconductor devices and piezoelectric vibrators comprise a case having an opening through which an electronic component housing cavity defined therein opens in an upper surface thereof, and a lid brazed to a portion of the case around the opening to close the opening of the case for sealing the cavity, as disclosed in Japanese Unexamined Patent Publication No. 2000-3973 (Patent Document 1).
The case is mainly composed of a ceramic material such as alumina or aluminum nitride. On the other hand, the lid comprises a base layer composed of a low thermal expansion metal such as a Fe—29% Ni—17% Co alloy (trade name: KOVAR), and a brazing material layer of a metal brazing material provided on one surface of the base layer. A silver brazing alloy mainly comprising silver is typically used as the metal brazing material.
As disclosed in Patent Document 1, seam welding is often employed as a method for brazing the lid to the portion of the case around the opening. The brazing can efficiently be achieved by the seam welding without the need for a vacuum environment with the use of a relatively simple facility. Another method for the brazing is electron beam welding, where in an electron beam is applied to an outer peripheral portion of the back surface (outer surface) of the lid stacked on the case to fuse the brazing material layer.
Japanese Unexamined Patent Publication No. HEI3(1991)-283549 (Patent Document 2) discloses an electronic component package, as a related art, which includes a copper foil provided between an aluminum nitride substrate and a cap or a cap fixture composed of a low thermal expansion metal such as KOVAR, the copper foil being brazed to the substrate and to the cap or the like.
Further, Japanese Unexamined Patent Publication No. 2000-164746 (Patent Document 3) discloses a lid material for a lid of an electronic component package, the lid material including a soldering material layer press-bonded to a Ni-based metal layer diffusion-bonded to a base layer, the Ni-based metal layer having a predetermined maximum-to-minimum thickness ratio.
With the height reduction and size reduction of the electronic components in recent years, it has been desired to further reduce the height and size of the packages for the electronic components. Therefore, the thickness of the lid is reduced. Further, the size of the case per se is reduced, and the thickness of walls defining the electronic component housing cavity in the ceramic case is correspondingly reduced.
In view of this, where the lid is brazed to the case with the use of the silver brazing alloy, since the silver brazing alloy has a relatively high melting point of about 780° C., in the case of the art of Patent Document 1, a great thermal stress occurs in the walls of the case during the cooling of the brazing material due to a difference in thermal expansion coefficient between the ceramic material constituting the case and the low thermal expansion metal constituting the base layer of the lid. The thermal stress causes cracks in the walls, resulting in the problem of reduced hermeticity. This problem is associated with not only the seam welding but also the electron beam welding.
The art disclosed in Patent Document 2 employs the copper foil as a material for accommodating the deformation of the cap or the cap fixture, and states that the copper foil preferably has a thickness of 0.1 to 2.0 mm. The copper foil may be effective for prevention of the breakage of the planar substrate, because the copper foil per se thermally deforms to a greater extent in thicker regions thereof. However, a sufficient effect cannot be provided for prevention of the breakage of the thin walls of the case having a reduced size. It is also stated that the copper foil is brazed to the substrate and to the cap or the like. This results in very poor brazing workability, thereby leading to lower package productivity.
In the art disclosed in Patent Document 3, on the other hand, the Ni-based metal layer and the soldering material layer stacked on the Ni-based metal layer are merely press-bonded to each other, but not diffusion-bonded to each other. Therefore, where a brazing material such as a silver brazing alloy having an inferior cold press-bonding property is employed instead of the soldering material, a bonding strength between the Ni-based metal layer and the brazing material layer is problematic. The layers may be diffusion-annealed after the press-bonding thereof, but the problem of voids occurring during the diffusion-annealing is not disclosed nor suggested at all.
In view of the problems described above, it is an object of the present invention to provide a lid for an electronic component package which, when being brazed to a case of the electronic component package, alleviates a thermal stress occurring in the case thereby to reduce the possibility of the breakage of the case and ensures excellent brazing workability, and to provide a lid material for the lid and a production method for the lid material. It is another object of the present invention to provide an electronic component package sealed by the lid with excellent hermeticity.